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Regional Geochemistry of Tertiary Igneous Rocks in Central Chile: Implications for the Geodynamic Environment of Giant Porphyry Copper and Epithermal Gold Mineralization

Peter Hollings^,* and David Cooke

Centre for Ore Deposit Research, University of Tasmania, Private Bag 79, Hobart, Tasmania, Australia 7001

Alan Clark

Department of Geological Sciences and Geological Engineering, Miller Hall, Queen’s University, Kingston, Ontario, Canada K7L 3N6

Corresponding author: e-mail, peter.hollings{at}lakeheadu.ca

The giant porphyry copper-molybdenum deposits of central Chile formed within a thick sequence of Cretaceous to Pliocene volcanic rocks. The predominantly calc-alkaline basaltic andesites of the Cretaceous Las Chilcas Formation are characterized by La/Sm_n ratios of 1.8 to 2.5 and Sm/Yb_n of 1.8 to 2.8. The upper Oligocene to lower Miocene Abanico Formation (variously defined as the Los Pelambres, Abanico, or Coya Machalí Formations) ranges from basalt to rhyolite in composition and exhibits a broad southward transition from calc-alkaline to tholeiitic. All samples from this formation are characterized by LREE enrichment and moderately, or locally strongly, fractionated HREE (La/Sm_n = 1.3–4.1; Sm/Yb_n = 1.5–5.8). The basaltic andesites and andesites of the middle Miocene Salamanca Formation have REE chemistry similar to that of the Upper Cretaceous strata (La/Sm_n = 1.5–2.7; Sm/Yb_n = 1.6–2.7). The overlying middle Miocene Farellones Formation ranges from tholeiitic to calc-alkaline and from basalt to andesitic and has a similar LREE enrichment and fractionated HREE (La/Sm_n = 1.7–2.5; Sm/Yb_n = 1.7–3.3). The Pliocene La Copa Rhyolite Complex, however, is strongly LREE enriched and HREE depleted (La/Sm_n = 3.8–3.9; Sm/Yb_n = 4.2–4.7). Overall, the trace element geochemistry of the Cretaceous to middle Miocene volcanic rocks is characterized by enriched LREE and negative Nb anomalies, consistent with an arc setting, with only minor differences in the abundance of most elements. Crustal thickening during the Miocene in central Chile has been suggested to have been responsible for a transition from an amphibole- to garnet-dominated residual mineralogy resulting in the release of fluids that enabled the formation of giant copper porphyry deposits. However, the gradual increase in La/Yb through the early, middle, and late Miocene reported in earlier studies and interpreted to be a response to crustal thickening is not observed in the regional data. Instead, a rapid change in the geochemical signature between the end of the eruption of the Farellones Formation and the eruption of the high La/Yb La Copa Rhyolite Complex implies a more abrupt change in the tectonic environment. Isotopic data broadly support this, although lower _Nd values in the Farellones Formation imply a greater role for crustal contamination in younger suites. In the absence of gradual crustal thickening, it is suggested that the subduction of the Juan Fernández Ridge may have been the key geodynamic process responsible for the genesis of the three middle Miocene to lower Pliocene giant porphyry copper deposits in central Chile, possibly by promoting crustal-scale faulting and even acting as a source of metals.

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